Typically, the analysis or testing of blood for the presence of glucose or other materials is carried out by applying a droplet of the blood to a test strip which carries a pad of a mixture of reagents which give a colour indication in response to one or more of the materials under test. The test strip typically carries the reagent(s) in a gelatin or other inert polymer or gel matrix pad at one end of a white plastic strip. However, this method suffers from the problems of contamination both of the sample by airborne and other materials and from the risk of cross-contamination of the samples on the tests sticks where an operator is handling a number of tests simultaneously. It is also necessary to remove excess blood sample to enable the colour developing in the reagent pad to be observed, and this may lead to rupture or smearing of the reagent pad. There is also the problem of disposing of the bloodied test stick after the test has been completed.
It has been proposed, in for example U.S. Pat. No. 4,935,346, to apply the blood sample to one side of a translucent porous membrane carrying a reagent mixture within the pores so that the plasma of the blood flows into the pores and reacts with the reagent to develop a colour which is then observed from the other side of the membrane as it develops. Such a technique will be denoted hereinafter as a back reading technique.
However, we have found that if the pores are of a size which can be readily achieved with conventional manufacturing techniques, the surface tension effects at the entry to each pore passage are so great that the wall of a blood cell in contact with the face of the membrane is ruptured and the colour visible from the other face of the membrane is distorted by the presence of red cell wall fragments which have penetrated into the pores. It is therefore necessary to take extra colour readings to compensate for this distortion, which adds to the complexity of the process and the cost of devices for use therein.
It has been proposed to apply a surface coating to the membrane, including the internal walls of the pores thereof, so as to increase the cell protein binding properties of the membrane material. However, this treatment does not overcome the cell wall rupture problems described above and fragments of the cell wall still pass through the membrane to affect the colour observed.
We have now devised a means by which this problem can be reduced and by which a membrane suitable for use in the back reading technique can be produced which does not cause significant rupture of the blood cell walls. In our invention the pores which are intially present in the membrane are blocked or blinded so that there is no free flow of fluid into the pores and the fluid component to be tested is separated from cellular components of a material to be tested by the membrane. In this way, the membrane can be mounted as an end wall of a chamber into which the blood is introduced for testing. The sample of blood is thus held enclosed within the chamber and the risk of external contamination and cross-contamination from other samples is reduced. Also, since the sample is retained within the chamber, the problems associated with disposal of the sample after testing are reduced.